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Organocopper compounds in organometallic chemistry contain carbon to copper chemical bonds. Organocopper chemistry is the science of organocopper compounds describing their physical properties, synthesis and reactions. They are reagents in organic chemistry.
The first organocopper compound, the explosive dicopper acetylide Cu2C2 was synthesized by Bottger in 1859. Henry Gilman prepared methylcopper in 1936. In 1941 Kharash discovered that reaction of a Grignard reagent with cyclohexenone in presence of Cu(I) resulted in 1,4-addition instead of 1,2-addition. In 1952 Gilman investigated for the first time dialkylcuprates.
Organocopper compounds are very reactive towards oxygen and water forming copper(I) oxide, tend to be thermally unstable and are generally insoluble in inert solvents. They are therefore difficult to handle and of little practical value. On the other hand organocopper reagents are used very frequently in organic chemistry as alkylating reagents prepared in situ in an inert environment with in general more functional group tolerance than corresponding Grignards or organolithium reagents. The electronegativity of copper is much higher than its next-door neighbour in the group 12 elements, zinc, suggesting less nucleophilicity for carbon.
Copper belongs to the group of coinage metals together with silver and gold and their chemistries have many similarities. The oxidation state can be +1 or +2 and intermediates can have oxidation state +3. Monovalent alkylcopper compounds (R-Cu) form divalent cuprates R2CuLi with organolithium compounds (R-Li) now known as Gilman reagents. Organocopper compounds can be stabilized with organophosphanes (R3P).
The cuprates have complex aggregation states in crystalline form and in solution. Lithium dimethylcuprate is a dimer in diethyl ether forming an 8-membered ring with two lithium atoms coordinating between two methyl groups.
The first ever crystal structure was determined in 1972 by Lappert for CuCH2SiMe3. This compound is relatively stable because the bulky trimethylsilyl groups provide steric protection. It is a tetramer forming an 8-membered ring with alternating Cu-C bonds. In addition the four copper atoms form a planar Cu4 ring based on three-center two-electron bonds. The copper to copper bond length is 242 pm compared to 256 pm in bulk copper. In pentamesitylpentacopper a 5-membered copper ring is formed and pentafluorophenylcopper is a tetramer.
With carbon monoxide copper forms a non-classical metal carbonyl.
In many organometallic reactions involving copper, the reaction mechanism invokes a copper intermediate with oxidation state +3. For instance, in reductive elimination processes, Cu(III) is reduced to Cu(I). However Cu(III) compounds are rare in chemistry in general and until recently organocopper(III) species have been elusive. In 2007 the first spectroscopic evidence was obtained for the involvement of Cu(III) in the conjugate addition of the Gilman reagent to an enone :
In a so-called rapid-injection NMR experiment at -100°C, the Gilman reagent Me2CuLi (stabilized by lithium iodide) was introduced to cyclohexenone (1) enabling the detection of the copper - alkene pi complex 2. On subsequent addition of trimethylsilyl cyanide the Cu(III) species 3 is formed (indefinitely stable at that temperature) and on increasing the temperature to -80°C the conjugate addition product 4. According to an accompanying in silico experiments  the Cu(III) intermediate has a square planar molecular geometry with the cyano group in cis orientation with respect to the cyclohexenyl methine group and anti-parallel to the methine proton. With other ligands than the cyano group this study predicts room temperature stable Cu(III) compounds.
Organocopper reactions are classified in a number of reaction types:
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Organocopper_compound". A list of authors is available in Wikipedia.|